1. Technical Field
The invention relates to a solid-state imaging device including a photoelectric conversion section in which a plurality of photoelectric conversion elements for detecting pixels forming an image are arranged in a one-dimensional or two-dimensional manner, and a driving method for the solid-state imaging device.
2. Description of the Related Art
A solid-state imaging device having a plurality of photoelectric conversion elements arranged in a two-dimensional manner is basically configured as shown in FIG. 17, for example. That is, the solid-state imaging device includes a silicon substrate 301, photodiodes (PD) 302 serving as photoelectric conversion elements, vertical charge transfer sections (VCCD) 303, charge read regions (TG) 304, a horizontal charge transfer section (HCCD) 305, and a signal output amplifier 306. In addition, each of the vertical charge transfer sections 303 and the horizontal charge transfer section 304 are shift registers that can transmit analog signals using CCDs (charge coupled devices).
The plurality of photodiodes 302 are arranged on a surface of the silicon substrate 301. For example, the photodiodes 302 are arranged at intersections of a square lattice. That is, the plurality of photodiodes 302 arranged in the two-dimensional manner form a two-dimensional photoelectric conversion section. In addition, an optical color filter that each allows only a corresponding color component, such as ‘R’, ‘G’, and ‘B’, to pass therethrough is disposed on a light receiving surface of each photodiode 302, so that each of the photodiode 302 detects the corresponding color component. Each of the photodiodes 302 performs photoelectric conversion and generates an electric signal corresponding to an amount of electric charges, which is determined according to the intensity of received light, the length of exposure time, or the like.
Each vertical charge transfer section 303 is provided for each column so as to be positioned adjacent to the photodiodes 302 in each column. Each of the vertical charge transfer sections 303 extends in the vertical direction, that is, in the longitudinal direction (direction indicated by arrow ‘Y’) in FIG. 17.
Electric charges generated by the photodiodes 302 are transferred to the vertical charge transfer sections 303 through the charge read regions 304, sequentially transferred through the vertical charge transfer sections 303 in the direction indicated by the arrow Y After reaching the horizontal charge transfer section 305, the electric charges are sequentially transferred through the horizontal charge transfer section 305 in the direction indicated by arrow X, and then output from the signal output amplifier 306 as electric signals in pixel unit.
FIG. 18 is a view illustrating a cross-sectional structure near one photodiode 302. As shown in the drawing, the charge read region 304 is formed at the position adjacent to each photodiode 302 disposed on the silicon substrate 301, and the vertical charge transfer section 303 is formed at the position adjacent to the charge read region 304. The vertical charge transfer section 303 is formed with a transfer channel 311 and an electrode 312. By applying a predetermined voltage (read pulse) to the electrode 312, an electric potential is applied to the charge read region 304, and thus a charge signal generated by the photodiode 302 can be transferred (read out) to the transfer channel 311. Since a plurality of independent electrodes including the electrode 312 are arranged on the transfer channel 311 along the direction indicated by arrow Y, the charge signal in the transfer channels 311 can be transmitted in the arrow Y direction by applying a voltage to the electrodes.
In recent years, it is demanded to photograph a high-resolution image. Accordingly, in order to respond to such demands, it is necessary to increase the number of photoelectric conversion elements provided in a photoelectric conversion section of a solid-state imaging device. However, in the vertical charge transfer section 303 or horizontal charge transfer section 305 described above, signal charges generated in photoelectric conversion elements are sequentially transferred in pixel unit in synchronization with a predetermined transfer pulse. Accordingly, if the number of photoelectric conversion elements increases, time taken until signal charges of all pixels are completely transferred becomes long. As a result, time required for imaging also becomes long.
For this reason, it has been tried to divide a photoelectric conversion section of a solid-state imaging device into a plurality of regions and to transfer signal charges independently for every divided region in the relate art. For example, in a technique disclosed in JP 2004-364235 A, a plurality of horizontal transfer sections corresponding to the above-mentioned horizontal charge transfer section 305 are provided in accordance with area division of a photoelectric conversion section.
For example, in a solid-state imaging device shown in FIG. 19, it is assumed that a photoelectric conversion section 300 is divided into two parts and that two horizontal charge transfer sections 305(1) and 305(2) are provided. In this case, since signal charges of the divided regions can be transferred simultaneously (in parallel) using the two horizontal charge transfer sections 305(1) and 305(2), the pixel number of signal charges that each horizontal charge transfer section is to transfer becomes half of the total number. Accordingly, signal charges may be completely transferred within half of normally required time.
However, since the signal output amplifier 306 from which electric signals are output in the pixel unit is provided at the downstream end of the horizontal charge transfer section 305 as described above, it is necessary to prepare a plurality of signal output amplifiers 306 as well as the horizontal charge transfer sections 305 as shown in FIG. 19, in order to simultaneously transfer the signal charges of a plurality of regions.
However, it is difficult to form a plurality of signal output amplifier 306(1) and 306(2) whose electrical characteristics are equivalent to each other, on one semiconductor substrate. Therefore, in the solid-state imaging device having the configuration shown in FIG. 19, a difference in characteristics of the signal output amplifiers 306(1) and 306(2) causes a conversion gain difference, a linearity difference, and an offset (black level) difference, appearing in a photographed image. That is, in the case of creating a one-frame image by mixing a signal of each pixel output from the signal output amplifier 306(1) and a signal of each pixel output from the signal output amplifier 306(2), a brightness difference, coloring, or black floating at the time of darkness may occur in the resultant image. Particularly at a boundary position in a middle portion of the image frame, unnecessary line-shaped noises are generated, which significantly deteriorates the quality of an image.
The deterioration of the image quality described above may be suppressed, for example, by performing correction processing for multiplying either the signal of each pixel output from the signal output amplifier 306(1) or the signal of each pixel output from the signal output amplifier 306(2) by a preset constant. However, in the case of performing such correction using a constant, it is not possible to meet the change of an imaging environment.
For example, even if characteristics are accurately corrected immediately after switching on an imaging apparatus, ambient temperature changes, or a solid-state imaging device or other electronic devices positioned around the solid-state imaging device generate heat due to continuous imaging. As a result, temperatures of the signal output amplifiers 306(1) and 306(2) of the solid-state imaging device change. Then, since electrical characteristics of the signal output amplifiers 306(1) and 306(2) change differently, an error occurs in a result of the correction processing. As a result, it is not possible to avoid that the image quality deteriorates with time.
Therefore, JP 2004-364235 A proposes a configuration in which common reference charges generated by a special injection charge generating unit are injected into the two horizontal charge transfer sections and variation of characteristics is corrected using the reference charges.
Further, JP 2005-151079 A (corresponding to US 2005/0111061 A) proposes a technique for using an amount of light incident from an actual photographic subject as a signal for correction and for making an amount of light incident on a plurality of divided regions equal by using a blurring member, such as a frosted glass, when detecting the amount of incident light.
However, in the technique disclosed in JP 2004-364235 A, the reference charges additionally generated as well as signal charges of a photographed image should be injected into the respective horizontal charge transfer sections. Accordingly, since the number of components included in a solid-state imaging device increases, a structure thereof becomes complicated. In addition, since the same reference charges are not always injected into the plurality of horizontal charge transfer sections, exact correction may not be performed.
In addition, in the technique disclosed in JP 2005-151079 A (corresponding to US 2005/0111061 A), although correction is performed using the amount of light that is actually incident from a photographic substrate, the amount of light input to the plurality of horizontal charge transfer sections as a signal for correction does not correspond to an amount of original light from the same photographic subject. Accordingly, the correction cannot be performed precisely.